An active matrix liquid-crystal display (AMLCD) is a type of flat-panel display which currently is the overwhelming choice of notebook computer and smart-phone manufacturers because of its light weight, very good image quality, wide variety of colors, and fast response time.
Most commonly, an AMLCD display is a multilayered structure that comprises a light-guide panel (LGP) that emits white light; a matrix of color light filters; two polarization layers; and a liquid-crystal matrix that contains a plurality of liquid-crystal cells and is combined with a matrix of thin-film transistors (TFT).
Optical response to light passing through each liquid crystal cell results from the electrical drive of each cell of the TFT matrix. Each color liquid-crystal cell should contain at least three cells (subpixels) of liquid crystals. Each subpixel is covered by specific color light filters, e.g., red, green, or blue filter. Each pixel may contain more than three subpixels, which can be organized into different geometrical patterns. Since the cells of the liquid-crystal matrix are driven independently, at any given time some pixels may maintain an electrical state while others are in the stage of updating.
Such a display provides much brighter, sharper images than a display of the same size with a passive matrix. Further improvement of AMLCDs may be possible only with introduction of laser light sources. However, AMLCDs based on laser light sources do not yet exist on the market.
Advantages of lasers as light sources for active LCDs in comparison with conventional sources or now incoming LEDs are well known because laser illumination devices provide higher brightness, greater image contrast, wider variety of colors, smaller dimensions, and better performance efficiency. In spite of these advantages, laser illumination devices have not yet achieved widespread application because of a fundamental phenomenon that leads to microscopic image degradation, i.e., observation of a floating granular pattern in front of the image plane. This pattern is known as a “speckle” pattern caused by interference of light waves having different phases and amplitudes but the same frequency. The interaction of these waves produces a resultant wave. The amplitude and intensity of the resultant wave varies randomly.
The phenomenon of speckle formation can be explained as follows. When the surface of an object is illuminated with coherent light, e.g., laser light, each point of the illuminated surface acts as a secondary point of light source that reflects (transmits) and scatters a spherical wave. However, since the illuminated surface, itself, has its own surface microstructure, these waves will have different phases and amplitudes. More specifically, in the majority of cases, the light-reflecting or light-permeable surface that constitutes an object of illumination has surface roughness that is comparable to the wavelength of the illumination light. The main contribution to scattering of light is assumed to be the small portions of a surface that are irregularly arranged and that possess light reflection or refraction properties. With an increase in the steepness of roughness and the size of the illuminated area, the number of light illuminating points is increased. Propagation of such reflected (transmitted) light to the point of observation leads to interference of dephased but coherent waves at that point. As a result, the viewer sees a granulated or speckled pattern. In other words, speckles comprise an interference picture of irregular wavefronts that is formed when coherent light falls onto a rough surface or a surface that contains micro nonuniformities. In general, a speckled picture is formed when coherent light propagates in the space and meets micro nonuniformities on the way.
Thus, the phenomenon of speckle formation essentially restricts the scope of application of laser illumination devices in fields such as active displays, microscopy, optical metrology, optical coherent tomography, etc. For example, speckles are considered to be a problem in laser-based display systems such as laser television.
Quantitatively, speckles are usually evaluated by the amount of speckle contrast. Speckle contrast is reduced by creating many independent speckle patterns that are averaged in the eye retina or in a detector. Speckle contrast reduction can be achieved by various methods, such as changing an illumination angle, using different polarization states, using laser sources of close but still different wavelengths, using rotating diffusers that destroy the spatial coherence of laser light, or moving or vibrating light transmitting/reflecting membranes that are placed in the optical path of the illumination light.
Heretofore, laser-illuminated displays with pixilated image formation did not exist in the industry. However, the principle of design and operation of such displays are described in Pending U.S. patent application Ser. Nos. 13/317,544 and 13/331,261 filed on Oct. 21, 2011 and on Dec. 20, 2011, respectively, by the same applicants.